Symptoms of abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid rhythm being referred to as a tachycardia. The present invention is concerned with the treatment of tachycardias that are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of the chambers of a heart. The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper chamber to the lower chamber necessary for performing normal systole and diastole function. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissues within the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated.
The impedance usually rises at the tissue contact site when RF energy is delivered through an electrode. To create a deeper and larger lesion, the surface of the tissue contact sites needs to maintain a proper temperature by a cooled fluid irrigation or infusion to partially compensate for the temperature rise due to heat reflection from the lesion site following a RF energy delivery. The following U.S. patents disclose use of irrigation ports in different manners to cool the tissue contact surface. Those patents are U.S. Pat. No. 5,545,161 to Imran, U.S. Pat. No. 5,462,521 to Brucker et al., U.S. Pat. No. 5,437,662 to Nardella, U.S. Pat. No. 5,423,811 to Imran et al., U.S. Pat. No. 5,348,554 to Imran et al., and U.S. Pat No. 5,334,193 to Nardella. In practice, the fluid coming out of the irrigation ports may not evenly cover all the surface area of the electrode or the tissue to be ablated. Furthermore, none of the above discloses an irrigation system of cooled fluid through a rotatable electrode means to form a uniform protective fluid layer around the electrode.
The tip section of a catheter is referred to herein as the portion of that catheter shaft containing at least one electrode. In one embodiment, a catheter utilized in the endocardial radiofrequency ablation is inserted into a major vein or artery, usually in the neck or groin area. The catheter is then guided into an appropriate chamber of the heart by appropriate manipulation through the vein or artery. The tip of a catheter must be manipulatable by a physician from the proximal end of the catheter, so that the electrodes at the tip section can be positioned against the tissue site to be ablated. The catheter must have a great deal of flexibility in order to follow the pathway of major blood vessels into the heart. It must permit user manipulation of the tip even when the catheter body is in a curved and/or twisted configuration. A guiding catheter may be used to introduce the ablation catheter to near the lesion site.
The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large electrode about 4 to 8 mm in length for ablation purpose. The lesion is generally not deep because of potential impedance rise of the tissue in contact with the "stationary" catheter electrode and thereafter the ablation time needs to be cut short. The word "stationary" means that the contact point of the electrode with the tissue is the same point unless the electrode is rotatable or movable so that the contact point changes from time to time. In some cases, the contact of a stationary electrode of the conventional catheter with tissues reportedly results in potential tissue adhering to said electrode. A rotatable electrode is in need to reduce the tissue contact impedance rise and temperature rise by slightly moving the rotatable electrode around in a micro-moving manner so that the temperature rise is decreased by the surrounding fluid or by the irrigation fluid. Even in the case of a conventional catheter having irrigation capabilities by utilizing an irrigation port, the cooled fluid does not evenly and uniformly rinses the electrode, because the electrode is not rotatable and the electrode-to-tissue contact point is not accessible to the irrigation fluid.
After the exact location of a target tissue is identified, the ablation catheter may still not easily approach the target site even with assistance of an internal viewing means. This viewing situation may turn into a nightmare when an internal viewing approach becomes prohibitive or unavailable during procedures. An external ultrasonic imaging capability therefore becomes in need so that ablation is not taking place in an inappropriate location. The fluoroscope time can be substantially cut short when an external ultrasonic imaging is used instead. In the U.S. Pat. No. 4,794,931, there has been disclosed a catheter and system which can be utilized for ultrasonic imaging. However, there is no disclosure to how such a catheter and system can be utilized in conjunction with an endocardial or epicardial ablation catheter having a rotatable electrode with irrigation capabilities to achieve the desired ultrasonic imaging and ultimately the desired ablation.
Avitall in the U.S. Pat. No. 5,242,441 teaches a rotatable tip electrode. Said electrode is secured to a high torque wire for rotation and electrical conductivity. The tissue contact site is always the same spot even the electrode is rotated. The potential coagulum at the contact spot due to impedance rise would not go away because of its relatively stationary position of the rotatable tip electrode and absence of fluid irrigation to the electrode-to-tissue contact site.
After an ablation catheter is positioned at the desired location, a rotatable ball-type electrode can be moved axially along the distal section of the catheter shaft to create a long linear lesion without dragging the catheter. This can be achieved by a movable ball electrode of the present invention.
While a radiofrequency electrophysiology ablation procedure using an existing catheter has had promising results, the tip section of a known catheter usually has a fixed non-rotatable electrode and a fluid infusion port which may not evenly rinse the electrode when contacting the tissue for ablation purpose. Therefore there is a need for an improved catheter and methods for making a deeper and larger lesion in the cardiac tissue.